AD8217
Rev. A | Page 10 of 16
THEORY OF OPERATION
AMPLIFIER CORE
In typical applications, the AD8217 amplifies a small differential
input voltage generated by the load current flowing through
a shunt resistor. The AD8217 rejects high common-mode vol-
tages (up to 80 V) and provides a ground-referenced, buffered
output that interfaces with an analog-to-digital converter (ADC).
Figure 24 shows a simplified schematic of the AD8217.
LDO
LOAD
R2
R1
V
2
I
LOAD
V
1
AD8217
R4
R3
+IN
GND
–IN
SHUNT
OUT
4.5V
TO
80V
09161-024
Figure 24. Simplified Schematic
The AD8217 is configured as a difference amplifier. The
transfer function is
OUT = (R4/R1) × (V1 − V2)
Resistors R4 and R1 are matched to within 0.01% and have
values of 1.5 MΩ and 75 kΩ, respectively, meaning an input
to output total gain of 20 V/V for the AD8217.
The AD8217 accurately amplifies the input differential signal,
rejecting high voltage common modes ranging from 4.5 V to 80 V.
The main amplifier uses a novel zero-drift architecture, providing
the end user with an extremely stable part over temperature.
The offset drift is typically less than ±100 nV/°C. This perfor-
mance leads to optimal accuracy and dynamic range.
INTERNAL LDO
The AD8217 includes an internal LDO, which allows the device
to power directly from the common-mode voltage at the inputs.
No additional standalone supply is necessary, provided that the
common-mode voltage at the +IN pin is at least 4.5 V and up to
80 V. Once the common-mode voltage is above 5.6 V, the LDO
output reaches its maximum value, that is 5.6 V. This is also the
maximum output voltage range of the AD8217. Because the
AD8217 output typically interfaces with a converter, the 5.6 V
maximum output range ensures the ADC input is not damaged
due to excessive overvoltage.
The input bias current flowing through Pin +IN powers the
internal LDO and, therefore, doubles as the supply current
for the AD8217. This current varies depending on the input
common-mode voltage. See Figure 8 for additional information.
